As an optical disc 10, such as a CD (compact disc), DVD (digital versatile disc), and BD (Blu-ray disc), there is a playback-only optical disc having a spiral recording track TR which is formed by rows of pits TB with a track pitch TP on an information recording surface as shown in FIG. 1, and there is a write-once or rewritable type optical disc having a spiral recording track TR onto which recording marks TM are written with a track pitch TP on an information recording surface as shown in FIG. 2.
An optical disc apparatus has an optical head apparatus irradiating the optical disc 10 with laser light and detecting reflected light of the laser light and a servo circuit causing a light-collected spot at which the light is collected on the information recording surface of the optical disc to follow the recording track TR. As shown in FIG. 3, a photodetector 19 of a conventional optical head apparatus has a main-beam light-receiving unit 19M for detecting zero-order diffracted light (main beam) which is reflected by the optical disc, and sub-beam light-receiving units 19S1 and 19S2 for detecting positive and negative first-order diffracted light (sub beams) which is reflected by the optical disc. The main-beam light-receiving unit 19M is divided in four sections and includes divisional light-receiving elements 19a, 19b, 19c, and 19d. The sub-beam light-receiving unit 19S1 is divided in two sections and includes divisional light-receiving elements 19e and 19f. The sub-beam light-receiving unit 19S2 is divided in two sections and includes divisional light-receiving elements 19g and 19h. If detection signal values of the divisional light-receiving elements 19a, 19b, 19c, 19d, 19e, 19f, 19g, and 19h are A, B, C, D, E, F, G, and H, respectively, a main-beam push-pull signal MPP is expressed by the equation:MPP=(A+B)−(C+D),a sub-beam push-pull signal SPP is expressed by the equation:SPP=(E−F)+(G−H), anda differential push-pull error signal (tracking signal) TES is expressed by the equation:
  TES  =            MPP      -              k        ·        SPP              ⁢                  ⁢                  =                  (                  A          +          B                )            -              (                  C          +          D                )            -              k        ⁢                  {                                    (                              E                -                F                            )                        +                          (                              G                -                H                            )                                }                    where k is a coefficient (see Patent documents 1 and 2, for example).
Patent document 1: Japanese Patent Kokai Publication No. 61-94246
Patent document 2: Japanese Patent Kokai Publication No. 2005-346882